Gradient delay time

Figure 13 Timing diagram for the clean HMBC experiment with an initial second-order and terminal adiabatic low-pass 7-filter.42,43 The recommended delays for the filters are the same than for a third-order low-pass J filter. <5 and 8 are gradient delays, where 8 — <5 + accounts for the delay of the first point in the 13C dimension. The integral over each gradient pulse G, is H/2yc times the integral over gradient G2 in order to achieve coherence selection. The recommended phase cycle is c/)n = x, x, x, x 3 — 4(x), 4(y), 4( x), 4(—y) with the receiver phase c/)REC = x, x.

Figure 24 H2BC pulse sequence. Thin and thick bars represent 90° and 180° pulses, respectively, while, and the dashed boxes represent 13C decoupling, t = i2 = l/[ /max + Vminl T, = 0.5/[Vmln + 0.07(Vmax - Vmin)]. T3 = 0.5/[Vmax - 0.07(Vmax - Vmln)] and S = S + t(90H), where S is the gradient delay. T denotes the constant-time delay.

Fig. 5. Comparison of HMBC, D-HMBC and Field Gradient D-HMBC spectra of promoth-iocin B showing long range C- H couplings between methyl protons and relevant carbons of two oxazole units. Delay time was set to 500 ms. The sample (20 mg) was dissolved in...

Fio. 16. Schematic representation of the GERVAIS pulse sequence, identifying all pulse and delay timings. The pulse sequence shown is that for acquisition of successive velocity images. The magnitude and orientation of the g-gradient are changed as determined by the velocity or acceleration vector that is to be measured. 

Suppose that you have optimized a gradient on a 0.46 X 25 cm column and you want to transfer it to a 0.21 X 10 cm column. The quotient V2/V is ( nr2L)2/( nr1L), where r is column radius and L is column length. For these columns, V2/Vj = 0.083. Equation 25-8 tells us to decrease the volume flow rate, the sample mass, and the delay time to 0.083 times the values used for the large column. The gradient time should not be changed. 

Delay volume is the volume found between the time the gradient is started and when it reaches the column. The corresponding time is the delay time tD (see earlier). By running a gradient from pure methanol to methanol containing 100 ppm of UV-absorbing toluene, the delay volume can be measured. 

As an example, we assume a gradient from 100% water to 100% methanol in 20 minutes, on a column with a t0 value of 1.5 min. Now a solute that elutes with a retention time t = 15 min (t is the retention time under gradient conditions) is expected to yield fc=3 at the composition that was reached at the column inlet at r = 15 - 2x1.5 =12 min, which is 60% methanol, 40% water. Assuming that there is no delay time due to instrumental considerations, this is the composition at the start of the column, but not at the end. One and a half minutes (t0) later, this composition will have reached the end of the column. 

We can change the delay time of the G-BIRD to tune the isotope filter to different Vch values. The crucial /-coupling evolution for the time l//must give an inversion (180° rotation in the x -y plane from in-phase to antiphase and back to in-phase) in order for the coherence to survive the G-BIRD gradients. What happens to 13C-bound XH coherence if the delay is not exactly tuned to the /ch coupling We can use product operators to predict... 

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